Abstract Lateral ankle inversion sprains occur frequently in sports and recreation activities. Although many patients relatively recover after their first acute injury, chronic ankle instability is often reported after ankle sprains despite treatment. Chronic ankle instability includes mechanical instability and functional instability. Surgical procedure is effective in restraining the ankle joint to reduce mechanical instability. Functional instability (FAI), however, poses a more difficult issue with little improvement following either conservative treatment or surgical intervention. FAI is primarily identified by a condition of recurrent ankle sprain and/or an incidence of ankle giving way. There have been inconsistent results in the literature about the role of suggested FAI factors including ankle join laxity, proprioceptive deficiencies, peroneal muscle weakness, and elongated muscle response time. None of them provides a reasonable explanation for ankle giving way phenomenon. We have recently suggested an etiological FAI factor, i.e. hyper-reactivity to unloading reaction. We demonstrated in our previous study a hyper-reactivity to unloading reaction in FAI ankles compared to normal ankles in healthy subjects. A major limitation of our previous study was static stretch applied to the tested ankle. To further link the hyper-reactivity theory to FAI, we propose to test FAI ankles using a combination of dynamic stretch and nociceptive stimulation that mimic the condition of an ankle giving way incidence. In our recent pilot study we observed in the majority of the subjects a shifting in reactive movement pattern that is closely mimic the ankle giving way phenomenon. In the proposed study, we will test 25 FAI subjects and 25 healthy control subjects to demonstrate a shift in reactive movement pattern in the majority of the FAI subjects in terms of quantitative changes in kinematic and kinetic variables compared to ankles of healthy controls. The result of the proposed study may fill the last, but most important gap between our hyper-reactivity theory and FAI. PUBLIC HEALTH RELEVANCE: Narrative This study will help clinicians and researchers to better understand the mechanism of functional ankle instability. The results of the proposed study may lead to the development of more accurate diagnose tool and effective training approach for people who suffer from functional ankle instability.